1. Field of the Invention
This invention pertains generally to devices for controlling light intensity, and more particularly to a servo-control apparatus for liquid crystal windows that allows for continuous and predictable control of optical density.
2. Description of the Background Art
Liquid crystal window (LCW) technology has long been in use for switching a light source on and off. However, while LCWs are well suited to bi-state operation, the non-linearity and temperature dependence of their response does not facilitate repeatable, continuous control of optical density.
Alternative approaches include neutral density wedges. A wedge is a continuously varying neutral density filter which is superimposed on an identical wedge with a reversed density slope. The density of the superimposed area is thus flat, and is varied by mechanically sliding one or both wedges in opposite directions. However, wedges are slow and cumbersome compared to LCWs, and are typically over an order of magnitude more expensive.
Another known approach to continuous optical density control is to employ polarized light and mechanical positioning of optical components. This technique, however, has the same speed limitations and cost differential as the neutral density wedges.
Also, special forms of LCWs called "ferro-liquid crystal" (FLC) windows have been developed. These devices can be pulse-modulated to give a continuous density control. However, they are typically over an order of magnitude more expensive than conventional LCWs.
Therefore, there is a need for a high speed, low cost LCW-based apparatus that provides for repeatable, continuous optical density control. The present invention satisfies those needs, as well as others, and overcomes deficiencies found in prior devices.